The body cells depend upon products of both the respiratory and digestive systems' functions in order to maintain themselves. To produce energy, cells burn nutrient molecule fuel in oxygen. The digestive tract provides the nutrient molecules, through the process of digestion, while the respiratory tract provides oxygen. As such, the two systems work together to give your cells the ingredients they need to produce energy, which they use to communicate, build cellular products and grow.

The body cells depend upon products of both the respiratory and digestive systems' functions in order to maintain themselves. To produce energy, cells burn nutrient molecule fuel in oxygen. The digestive tract provides the nutrient molecules, through the process of digestion, while the respiratory tract provides oxygen. As such, the two systems work together to give your cells the ingredients they need to produce energy, which they use to communicate, build cellular products and grow.

Digestion actually begins in the mouth, as the enzymes in saliva begin to break down carbohydrate. As food is chewed, it becomes lubricated, warmer, and easier to swallow and digest. The teeth and mouth work together to convert each bite of food into a bolus that can readily move into the esophagus ("the food pipe").After the bolus is swallowed, it enters the esophagus.

The acidic environment of the stomach and the action of gastric enzymes convert the bolus into chyme, a liquefied mass that is squirted from the stomach into the small intestine. Carbohydrates tend to leave the stomach rapidly and enter the small intestine; proteins leave the stomach less rapidly; and fats linger there the longest.

The small intestine is the principal site of digestion and absorption. There, enzymes and secretions from the pancreas, liver, gallbladder, and the small intestine itself combine to break down nutrients so that they can be absorbed. The pancreas supplies enzymes to digest proteins, fats, and carbohydrates. Intestinal cells also supply some enzymes. The liver produces the bile required for the emulsification of fat, and the gallbladder stores the bile until it is needed. The absorption of nutrients in the small intestine is facilitated by tiny projections called villi, which provide more surface area for absorption. The nutrients pass through the intestinal membranes into the circulatory system, which transports them to body tissues. Nutrients are then absorbed into the cells.

Undigested chyme proceeds from the small intestine into the large intestine (colon), where it becomes concentrated, as liquid is absorbed in preparation for excretion. The remaining solid waste goes throught the rectum and out the anus. The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle. The right atrium is the upper chamber of the right side of the heart. The blood that is returned to the right atrium is deoxygenated (poor in oxygen) and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.